The present invention generally relates to an electrochemical reference electrode, more particularly to an electrochemical, second-order reference electrode for a half cell that exhibits superior potential stability and that is of the type having a metal/metal salt electrode surrounded by and in contact with an electrolyte. The electrolyte includes a soluble salt of the electrode metal cation, such soluble salt being present within the electrolyte in an amount sufficient to have the metal cation present within the electrolyte at its saturation concentration. The soluble salt and the anion react within the electrolyte to form a substantially insoluble salt which is the metal salt of and which also deposits onto the electrode in order to maintain the equilibrium condition of the metal/metal salt electrode.
Various systems and electrodes have been devised and used in order to measure and monitor ion activity within liquids such as human blood. Such systems include half cells or second-oorder eleectrodes, these systems tyupically including a measuring electrode that is particularly sensitive to the ion to be measured, as well as a refereence electrode that is provided in order to establish a substantially accurate and constant comparative potential. Significant progress has been made in order to provide reference electrodes that exhibit substantial improvements in accuracy and constancy of the comparative potential that they provide. One approach in this regard is described in United Kingdom published patent application No. 2,060,896, which proposes improving the potential stability of a second-order reference electrode by including substantially specific amounts of free silver of the colloidal type within the electrolyte of this type of an electrode.
Often, reference electrodes of this type are used within systems for measuring and/or monitoring the hydrogen ion concentration, or pH, of a liquid such as blood. Known systems in this regard employ an ion-selective electrode, such as an Ag/AgCl electrode, as the reference electrode. Difficulties that are usually encountered in connection with using an ion-selective electrode as the reference electrode include the fact that they are susceptible to measuring errors, such as drift, and can have an undesirably short service life as the electrode gradually moves toward an advanced state of decay.
Ideally, a reference electrode should provide a steady reference potential, particularly when used for in vivo measurement and monitoring of physiologically important ion concentration parameters such as pH. Preferably, these types of devices are sized and structured so as to be capable of being placed in vivo within the bloodstream, typically by means of a catheter structure. These types of structures are superior to electrode systems by which a probe is adhered to the patient's skin, which systems can be inherently less accurate and also difficult to apply in connection with continuous blood analysis in extra-corporal blood circuits such as those utilized during hemodialysis and open-heart surgical procedures.
Typically, the measuring electrode in these types of systems will include a component that is selectively sensitive in an electrically reactive manner to the parameter being measured, such as blood pH. Exemplary components in this regard include a measuring electrode that incorporates an ion-sensitive field effect transistor (ISFET) transducer having an ion sensitivity to hydrogen ions to determine hydrogen ion concentration, or pH.
In connection with in vivo measurement of ion concentrations within body liquids, it is extremely critical that the reference electrode function to provide an accurately constant comparative potential. Inaccuracies can arise, together with consequent measuring errors, due to design and structural shortcomings, despite the thoroughly enhanced structural and design characteristics of present-day reference electrodes.
The present invention provides a reference electrode that is suitable for use within a system for in vivo ion concentration measurement and monitoring, such as that carried out within the bloodstream of a patient, which reference electrode does not experience substantial drift and which does provide an accurately constant comparative potential. The reference electrode includes an electrode component of the metal/metal salt type that is surrounded by and in contact with an electrolyte including the metal cation of the electrode, which electrolyte has added thereto a substantially soluble salt of the electrode metal cation, such being added in an amount sufficient to have the metal cation present in the electrolyte at a saturation concentration. These metal cations within the electrolyte react with anions within the electrolyte in order to form a substantially insoluble salt that is the salt of the electrode and which salt also deposits onto the electrode.
It is accordingly a general object of the present invention to provide an improved reference electrode which reduces the likelihood of having errors occur during ion concentration measurements, especially in vivo measurements of ion concentrations within body liquids.
Another object of the present invention is to provide an improved reference electrode as a second-order electrode of considerably enhanced operational characteristics, especially a substantial reduction in drift and a considerable prolongation of the service life of the reference electrode.
Another object of the present invention is to provide an improved reference electrode that can be useful in exhibiting recovery of electrodes that have progressed to the point at which they have entered an advanced state of decay.
Another object of the present invention is to provide a reference electrode that includes a silver/silver chloride electrode member within an electrolyte solution, which electrolyte solution enhances the rapid achievement of and the maintenance of the equilibrium of the silver chloride composition of the electrode member.
Another object of the present invention is to provide an improved catheter system that is suitable for in vivo measurement of body liquids, which catheter system includes a reference electrode that provides an especially accurate constant comparative potential.
These and other objects, features and advantages of this invention will be clearly understood through a consideration of the following detailed description.